1. Field of the Invention
The present invention relates to backlight modules, and more particularly to a direct type backlight module typically used in a liquid crystal display (LCD).
2. Discussion of the Related Art
In a liquid crystal display device, liquid crystal is a substance that does not itself radiate light. Instead, the liquid crystal relies on light received from a light source in order to provide displaying of images and data. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.
Typically, a light source of a backlight module is one of the following two types: a cold cathode fluorescence lamp (CCFL), or a light emitting diode (LED). Disadvantages of a CCFL include high energy consumption, low optical uniformity, and poor purity of white light. In addition, after being repeatedly used over time, a brightness of the CCFL becomes degraded and a color of light emitted by the CCFL tends to shift. In general, the service life of a CCFL is about 15,000 to 25,000 hours. Furthermore, a CCFL only covers 75 percent of color space as defined by the National Television Standards Committee (NTSC). Therefore, using a CCFL cannot satisfy the requirements for a high quality color liquid crystal display. Unlike CCFLs, high powered LEDs can cover as much as 105 percent of color space as defined by the NTSC. In addition, these LEDs have other advantages such as low energy consumption, long service life, and so on. Therefore, high power LEDs are better suited for producing high quality color liquid crystal displays.
FIG. 6 illustrates a conventional backlight module 10 using a plurality of LEDs 12. The backlight module 10 includes a frame 11, an optical plate 14, and the LEDs 12. The frame 11 includes a base 112, and a plurality of sidewalls 114 extending from a periphery of the base 112. Top portions of the sidewalls 114 cooperatively form an opening 116 therebetween. The LEDs 12 are regularly arranged on the base 112 of the frame 11. The optical plate 14 is disposed on the frame 11 over the opening 116. Light rays emitted by the LEDs 12 are diffused in the optical plate 14, so that substantially planar light is outputted from the optical plate 14.
Each LED 12 includes a light output unit 121, and an optical lens 123 coupled to the light output unit 121. The optical lens 123 includes a light input surface 1231, a top surface 1233 opposite to the light input surface 1231, and a peripheral light output surface 1235 generally between the light input surface 1231 and the top surface 1233. Light rays emitted by the light output unit 121 enter the optical lens 123 through the light input surface 1231 and transmit to the top surface 1233. Many or most of the light rays undergo total internal reflection at the top surface 1233, and then exit the optical lens 123 through the light output surface 1235.
However, a significant proportion of the light rays still escapes from the optical lens 123 through the top surface 1233. This would ordinarily cause a bright area to occur in the optical plate 14 above the LED 12. In order to prevent this problem, the backlight module 10 further includes a transparent plate 13 disposed between the optical plate 14 and the LEDs 12. The transparent plate 13 defines a plurality of reflective layers 15 on a bottom thereof. The reflective layers 15 are positioned in one-to-one correspondence with the LEDs 12. However, precisely positioning the transparent plate 15 according to the LEDs 12 can be very problematic and troublesome, due to the small size of the LEDs 12. In addition, the transparent plate 13 makes the backlight module 10 rather heavy, and adds to manufacturing costs.
What is needed, therefore, is a new backlight module which can overcome the above-described shortcomings.